Full-Scale Fatigue Testing of the F/A-18 Fighter Aircraft

Interface's LowProfile™ load cell measuring the applied force from the actuator to the F/A-18 fuselage.

Boeing's 6th Gen F/A-XX fighter.

The F/A-18 is a twin-engine supersonic fighter jet capable of carrying explosive payloads while maneuvering through incredible pitch, yaw, and hyperbolic multi-rolls – all F/A models are designed to dogfight and attack ground targets. The initials “F/A” stands for Fighter/Attack.

The F/A-18 is used by the air forces of many different countries: but it is most notably recognized from the many air shows of the U.S. Navy's Flight Demonstration Squadron - the Blue Angels.

With a top speed just under Mach 2, this fighter can carry a wide variety of bombs and missiles, including air-to-air and air-to-ground, supplemented by the 20 mm M61 Vulcan cannon.

The F/A-18 is powered by two General Electric F404 turbofan engines, which give the aircraft a high thrust-to-weight ratio. It has excellent aerodynamic characteristics, primarily attributed to its leading edge extensions (LEX). Early fighters could depart from controlled flight if maneuvered very aggressively at high AOA with a heavy external weapons load. [2]

The U.S. Navy has sold the F/A-18 to many countries in the US export market. It has been purchased and is in operation at several foreign air services – purchased and modified by the foreign country military engineering staff to make modifications that best fit their military needs. [3]

Exported F/A-18’s are very similar to their U.S. model of a similar manufacture date, but some features, especially those features/components which are used in concert with aircraft carrier landings, are not typically sold to these foreign countries. Thus, since none of the foreign customers operate aircraft carriers, all export models have been sold without the automatic carrier landing system, and Royal Australian Air Force further removed the catapult attachment on the nose gear. [4]

Before an aircraft can take off for the 1st time, it has to prove its prowess on a hydraulic test bed. It must withstand a tough set of tests representing at least 2 - 3 aircraft lives with all conceivable "real life situations". For this purpose upwards of 100 hydraulic cylinders loading points have to be coordinated with a high degree of control effort.

Testing machine cylinders / actuators with Interface LowProfile™ load cells in the load string ensure exact measurements under "real" flight simulation, while also permitting sustained and highly precise reproducibility of all movements. This scenario takes place on a hydraulic test bed every day at several test facilities worldwide.[5]

Turbulence can cause the wing tips of the aircraft to flex over 10% or more perpendicularly to the length of the wing span. Observing the constant up-and-down movement of the wings spanning several meters in trial conditions, one gets a glimpse of the material stress to which these aircraft are subjected. Load cells specially design for fatigue testing (Interface Model 1000 LowProfile™), are installed in-line with the hydraulic cylinders that apply fatigue back-and-forth loading forces. This “time accelerated test” is carried-out over a period of 18 months or more to simulate the wing and fuselage in-flight movements / flexing sequences. The tests are engineered to represent a life-span of the aircraft lasting 25 - 30 years. [6] The advanced test rig with push-pull actuators installed under the fuselage was developed and set up with final results representing “The F/A-18 Full Scale Fatigue Test Project”, which corresponds to twice the service life of an F/A-18 fighter.

Experience from many years of operation of fighter aircraft, indicates that F/A-18 aircraft are subject to more damaging loading-per-flight-hour than most aircraft. And because the engineering designs vary from adopting country to adopting country, fatigue life has also varied significantly with these design changes.

After the selection of an F/A-18 fighter design and its engineering modifications, a thorough structural integrity study is initiated to address structural and material fatigue requirements – this typically resulting in the modification of several aircraft components, which must be retested in a fatigue-rated simulation.

Reinforced airframes must be certified for a service life of ≥5000 flight hours. To meet the rigid testing requirements, the structurally demanding fatigue test is performed in a controlled environment at various test laboratories. One such test facility is the RUAG Aerospace Research & Testing facility (as seen in the picture above). In this test, 68 hydraulic actuators, with their associated load cells in the load string, simulate possible ’real-world’ force loading encountered in flight. [7]

Interface has participate in most of these fatigue-rated tests, providing LowProfile™ fatigue-rated load cells capable of >1x108 fully-reversed load cycles. Every test without a single load cell failure in all the years we have supplied them to the many, many testing facilities. We are a proud company of American-made goods. We look forward to the continued success of the F/A-XX fighter models, the 6th generation announced by Boeing this past week. [8]